a(n) = 3*a(n-1) - a(n-2) - 3*a(n-3) + 2*a(n-4).

Terms

a(0) =1a(1) =3a(2) =5a(3) =13a(4) =27a(5) =59a(6) =121a(7) =249a(8) =503a(9) =1015a(10) =2037a(11) =4085a(12) =8179a(13) =16371a(14) =32753a(15) =65521a(16) =131055a(17) =262127a(18) =524269a(19) =1048557a(20) =2097131a(21) =4194283a(22) =8388585a(23) =16777193a(24) =33554407a(25) =67108839a(26) =134217701a(27) =268435429a(28) =536870883a(29) =1073741795